Method and apparatus for controlling cleanness of a heavy medium suspension



Nov. 1, 1966 P. w. CHASE ETAL 3,282,417

METHOD AND APPARATUS FOR CONTROLLING CLEANNESS OF A HEAVY MEDIUM SUSPENSION Filed Nov. l5, 1962 ma IL mw -HHr I United States Patent O 3,282,417 Y METHOD AND APPARATUS FOR CONTROLLING CLIANNESS OF A HEAVY MEDIUM SUSPEN- SIO Paul W. Chase, Mountain Iron, and Luther G. Hendrickson, Duluth, Minn., assignors to United States Steel Corporation, a corporation of Delaware Filed Nov. 13, 1962, Ser. No. 236,945 8 Claims. (Cl. 209-12) This invention relates to a method and Iapparatus for controllingthe cleanness of a medium used in a heavy medium minerals separation process.

In a conventional heavy medium process, mineral particle-s are Aintroduced to a vessel which contains a medium of specific gravity intermediate that of the values and gangue in the mineral. Commonly the medium is a water suspension of a finely divided magnetic substance, such as ferrosilicon or magnetite. If the mineral is an ore, such as iron ore, the values -sink while the gangue floats. The reverse -occurs with some materials, such as coal. After the sink and float products leave the separating vessel, the suspension is drained therefrom and recovered for re-use. Next the sink and oat products are Washed. The wash water, along with .a fraction of the suspension drained from the products, goes to a magnetic separator and thence to a densier. rI`he magnetic separator removes nonmagnetic contaminants and the densifier removes water to produce ka densified medium lof higher specic gravity than that used in the sepa-rating vessel. The densied medium joins the remainder of the :drained suspension 'and returns to the separating vessel. Water accompanying the mineral particles dilutes the medium in the vessel to the proper specific gravity. Reference can be made to Wade Reissue Patent No. 22,191, dated September 29, 1942, for a detailed showing of a process of this type, `although the specific pieces ofequipment Wade shows are not of the most modern construction.

To achieve `a maximum recovery of the values at the desired grade, the specific gravity and other characteristics of the medium in the separating vessel should be maintained as nearly constant as possible. Variations in the feed rate, walter content of the feed, and densifying c'haracteristics make it diflicult to maintain constant conditions. In another application Serial No. 236,944, tiled the same date as the pfresent application, we have shown and cl-aimed a method `and apparatus for automatically controlling the specific gravity of the medium and maintaining it substantially constant. Another characteristic which we nd desirable to control is the quantity of nonmagnetic contaminants .in the circulating medium or the cleanness of the medium. Such contaminants are removed only from the wash water and the fraction of the drained suspension which is treated in the magnetic separator. Hence the 'larger the treated fraction, the larger the'quantity of contaminants removed and the cleaner the medium. The viscosity of the medium varies with its cleanness.

Anobject of our invention is to provide a method and apparatus for automatically controlling the cleanness of Vthe circulating medium in a heavy medium minerals sep `aration process.

A further lobject is to provide a control method and apparatus of the foregoing type which operate in conjunction with a specific gravity control, whereby both the specific gravity and cleanness of the medium are controlled automatically.

A more spe'ciic object is to provide a method and apparatus for automatically controlling the cleanness of the circulating medium in which the extent of contamination is determined by measuring both the density and the quantity of magnetic particles in the medium, and the difference between these measurements is used to regulate Patented Nov. 1, 1966 ICC addition of water to the medium to decrease the content of contaminants.

In the drawing:

FIGURE 1 is a schematic tlowsheet of a heavy medium minerals separation plant equipped with our apparatus for controlling cleanness of the medium; and

FIGURE 2 is a perspective View of a preferred form of splitter embodied in our apparatus.

FLOWSHEET FIGURE 1 shows a owsheet of a heavy medium plant which includes a pump box 10, a medium-circulating pump 12, a separating vessel 13, and sink and float screens 14 and 15. Pump 12 continuously feeds the medium, a water suspension of magnetic particles, from the pump box to the -separating vessel 13. Mineral particles (for example 1% x M1 inch iron ore) also are introduced to this vessel. Heavier mineral parti-cles sink in lthe medium and discharge from lthe separating vessel to the sink screen 14, while lighter mineral particles oat and discharge to the float screen 15. Screens 14 and 15 have drain sections 14a Iand 15a and wash sections 14b and 15b over which the sink and float products pass in succession. Suspension drained from the sink product as it passes over the drain section 14a returns directly to the pump box 10. Suspension drained from the oat product as it passes over the drain section 15a goes to an adjustable splitter 16, which routes variable fractions to the pump box 10 `and 'to another pump box 17. Water is applied to both the sink and float products as they pass over the Wash sections 14b and 15b to wash away addition-al medium Wash Water from both products goes to the pump box 17, from which a pump 18 delivers Ithe contents to a magnetic separator 19 to remove nonmagnetic contaminants. The remaining magnetic particles and water go to a densicr 20, which removes water. Densied suspension from the densiiier returns to the pump box 10. In the example of an ore, sink particles leaving screen 14 usually are Aa finshed concentrate product, while oat particles leaving screen 15 and nonmagnetic contaminants from the magnetic separator 19 usually are a finished tailing product.

The individual pieces of apparatus other than our preferred splitter, `las well as the portion of the owsheet thus far described, are conventional and hence not descri'bed in greater detail. However, we point out that the densier is of a type in which both the speed and vdepth of the rake can be changed to increase or decrease the quantity of densified suspension feeding therefrom. We have illustrated a densier which has a rotating rake or screw 21 and a motor 22 for lowering or raising the screw. A densier =of this type, and also a suitable separating vessel and pumps, .are available commercially from Western Machinery Co., San Francisco, California, 'and are described in a printed publication by the supplier entitled Wemco Equipment for Heavy Media Separation, Bulletin No. Hl-BIZ. Another example of a suit- .able densifieris a reciprocating rake type of classifier 'available commercially from Dorr-Oliver Incorporated, Stamford, Connecti-cut, and is described in a printed pub- 'lication by the supplier, Bulletin 2281. We also point out that the plant may include other conventional pieces v of apparatus, such as demagnetizing coils for the recovered suspension, which we have not shown, since they are not involved in the present invention.

CLEANNESS CONTROL In accordance with our invention, we mount both a line is a little higher than that of the medium actually in the vessel and the content of contaminants a little lower, but both may be considered representative for control purposes. We connect a recorder-controller 25. to meter 23 to record the specific gravity of the medium. The specic gravity is of course proportional to the total solids content (magnetic particles plus nonmagnetic contaminants). We connect `another recorder 26 to meter 24 to record the content of magnetic solids in the medium. Recorders 25 and 26 transmit signals representative of the total solids content and the magnetic solids content respectively to a contamination recorder-controller 27. The last named recorder develops a signal representative of the difference between the total solids and the magnetic solids, which signal is representative of the content of contaminants in the medium or its cleanness We introduce water to the medium in the separating vessel 13 through an adjustable valve 28. The recorder-controller 27 automatically adjusts valve 28 to vary the volume of water yin accordance with the content of contaminants to maintain the content at a relatively constant set value; that is, the more contaminants, the more water we introduce.

SPECIFIC GRAVITY CONTROL A change in the volume of water introduced through valve 28 not only corrects the content of contaminants in the medium, but also changes the specific gravity from its set value. Therefore we use our cleanness control in conjunction with an a-utomatic control for the specific gravity, preferably similar to one of those shown in our aforementioned application Serial No. 236,944 or in tanother application Serial No. 236,946 tiled the same date by the co-inventor Chase jointly with Albert T. Koenen. We have illustrated a specific gravity control as shown in FIGURE 1 of the former application. The recordercontroller 25 transmits a signal representative of the density of the medium in vessel 13 to a splitter-positioner 31, which is mechanically connected to the splitter 16. The positioner 31 adjusts the splitter to vary the fraction of the drained suspension treated in the magnetic separator 19 and densiler 20 inversely wit-h changes in specic gravity. If the specific gravity of the medium goes down from its set value, the positioner adjusts the splitter to route la larger fraction of the drained suspension through v the magnetic separator and densier, whereupon larger quantities of nonmagnetic contaminants and water are removed from the circulating suspension and its specific gravity thus raised. The reverse action occurs if the specific gravity of the medium goes up from .its set value. After an adjustment is made in the splitter position, there would normally be a time lag of several minutes before medium yof corrected specic gravity reaches the separating vessel 13, but there is an immediate change in the level of medium in the pump box 10. If a larger fraction of the iloat drain product goes to the magnetic separator 19 and densier 20, less of course returns directly to the pump box and the level falls. The reverse action occurs if a smaller fraction goes to the magnetic separator. We connect a level-sensing device 32 to the pump box and connect a controller 33 to the level-sensing device. Controller 33 transmits `a signal representative of the level of medium in the pump box to a speed-changing device 34 for the mot-or which drives the densier screw 21. When the level falls `or rises from its set value, the screw immediately commences to turn faster or more slowly. As a result, particles actually in transit within the screw feed faste-r or more slowly to the pump box, and the specific gravity of the medium in the pump -box is corrected promptly. The speed change itself produces only :a temporary change in the feed rate. After the screw has fed all the medium particles Iactually in transit at the moment of the speed change, the speed change ceases to,be a factor, -but by this time the change caused by adjustment of the splitter takes effect. Hence medium in the pump box continues to be of the corrected specific gravity. f

The cleanness control and the specific gravity control complement each other. For example, if the mediu-m becomes more contaminated, the recorder-controller adjusts valve 28 to increase the volume of water introduced to vessel 13, thus lowering the specic gravity. The recordercontroller 25 adjusts the splitter 16 t-o route a larger fraction of the float drain product to the magnetic separator 19 and densier 20. The magnetic separator removes an increased quantity of contaminants from the circuit, while the densier removes lmore Water. If the circuit is properly adjusted, it stabilizes .at set values for both the contaminants and specic gravity.

As the plant operates, medium particles gradually are depleted. Hence the trend -is for splitter 16 to route a larger and larger fraction of the drained suspension to the magnetic separator 19 and densier 20 and for the densifer screw to run faster and faster. Preferably we connect the screw speed-changing device 34 with the raise-lower motor 22. When the speed of the densier screw approaches a set maximum or minimum, a signal for .a further increase or decrease in speed automatically operates motor 22 to lower or raise the screw. Our aforementioned application Serial No. 23 6,944 shows in detail one form of electric circuit We can employ `for this purpose. Since this feature is not essential to the cleanness control claimed in the present application, we have not repeated this showing.

SPLITTER FIGURE 2 shows structural `details of our preferred form of splitter 16. A rst fixed launder 46 receives suspension drained from the float product on screen 15. A second fixed launder 47 is located beneath launder 46 to receive the Ifraction of the suspension which returns directly to the pump box 10. A third fixed launder 48 is located beneath launder 46 and offset therefrom to receive the fraction which goes to the magnetic separator 19 and densier 20. The splitter includes a swinging launder 49 .and a pair of links 50 attached to this launder and pivoted to an overhead support 51. The swinging launder is located in the space between the fixed launders 46 and 47 and it has an outlet spout 52 in its end above the fixed launder 48. The positioner 31 is connected to the links 50 to move the swinging launder back and forth in response to signals from the recorder-controller 25. The splitter constructien per se is claimed in another application led by the co-inventor Hendrickson, Serial No. 361,498, led April 2l, 1964, now Patent No. 3,207,173, which is a `division of application Serial No. 208,757, iiled July .10, 1962, .and now Patent No. 3,235,079.

INSTRUMENTS The individual instruments Aused in our control apparatus are of conventional construction and available commercially. Hence we have not shown nor described them in detail, but instead reference can 'be made to printed publications for showings.

Considine, Process Instruments and Controls Handbook, published by McGraw-Hill Book Company, copyright 1957, Library of Congress Catalog Card No. 56-8l69, shows and describes instruments suitable for several for our purposes. Considine shows a recordercontroller (pages 11-22 or 1l-26) suitable yfor our recorder-controllers 25 and 27 and recorder 26, a cylinder-type operator with positioner (pages 10-27) suitable for our splitter-positioner 31, and a motor operated valve (pages 10-51) suitable for our valve 28. Suitable recordercontrollers also are available commercially from Leeds and Northrup Company, Philadelphia, Pa. as Type H. We use a Type `I-l recorder alone for our recorder 26. We add a CAT controller for our recorder-controller 25, and a PAT controller for our recorder-contoller 27. These instruments are described in printed publications by the supplier, Data Sheets ND46- 33(106)80-558 and ND46-51(100)60658 and Folder ND4(7b)80-1158 pages 5 and 6 PAT, pages 9 and 10 CAT. We equip recorders 25 4and 26 with retransmitting slide wires. A suitable positioner 31 also is available commercially from Foxboro Company, Foxboro, Mass. as the .Stabiload and is described in a printed publication by the supplier, Bulletin No. 446. A suitable motor for valve 28 is available commercially from Minneapolis- Honeywell Regulator Co., Minneapolis, Minn. as the Actionator M-930B and is described in a printed publication by the supplier Form No. 95-2624.

Our density meter 23 can -be an Ohmart cell .as shown in Ohmart Patent No. 2,763,790 or a gamma gage. A

suitable magnetic density meter 24 is available commercially from Ramsey Engineering Co., St. Paul, Minn., as the Model PC-6 Ramsey Pipe Coil used in conjunction with a Ramsey Converter. The pipe coil lgives an A.C. Voltage signal proportional to the content of magnetic solids, `and the converter changes this signal to a D.-C. voltage signal to operate the recorder. A suitable levelsensing device 32 and level controller 33 are available commercially from Fisher Governor Co., Marshalltown, Iowa, as Type 249P and Type 250() Fisher Level- Trol and are described in a printed publication 'by the supplier, Bulletin F-4A. A suitable screw speed changer 34 is available commercially from Louis Allis Co., Milwaukee, Wis., as the Select-a-spede drive and is described in a printed publication by the supplied, Service manual, Section 14A, July 1, 1957.

`Our illustrative recorder-controller 25 generates an electric signal. When We use this signal to control a pneumatically operated positioner 31, we include an electric-to-pneumatic transducer. A suitable transducer for this purposeis available commercially from Fisher Governor Co. as Type 543 and is described in a printed publication by the supplier, Bulletin E543. Our illustrative level controller 33 generates a pneumatic signal. When we use this signal to control the speed of the ldensiier motor, we include a pneumatic-to-electric transducer. A suitable transducer for the latter purpose is available commercially from vTaylor Instrument Companies, -as the Transcope Servomatic Transducer and is described in a printed publication by the supplier, Bulletin 98375, May 1960.

From the foregoing description, it is seen our invention affords a relatively simple yet effective method and apparatus for controlling the content of non-magnetic contaminants in the medium used in a heavy medium minerals separation process. Used in conjunction with a specific gravity control, our invention automatically holds both the specic gravity and the cleanness of the medium at relatively constant Values.

While we have shown and described only Ia single embodiment of the invention, it is apparent that modications may arise. Therefore, we do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

W-e claim:

1. In a heavy medium minerals separation process in which mineral particles .are introduced to a water suspension of magnetic parti-cles, heavier mineral particles sink in the s-uspe-nsion while lighter mineral particles float, the resulting sink and .il-oat products are successivelydrained of suspension and washed with water, both the wash water and .a variable fraction of the drained suspension are treated to remove nonrnagrretic contaminants andwater and thereby produce a idensilied suspension, said densiled suspension together with the remainder of the drained sus-pension is transferred to a pump box, and the suspension feeds from the pump box to la vessel where the mineral particles are introduced, the combination therewith of a method of controlling the cleanness of the suspension within .said vessel comprising determining the content of nonmagnetic contaminants in the suspension at a ylocation representa-tive of the vessel, introducing a variable volume of water to said vessel to maintain a relatively constant set content of contaminants in the suspension within said vessel, and adjusting the content of magnetic particles in the suspension entering said vessel after the water volume changes, thereby compensating for changes in the specific gravity of the suspension brought about by variations in the water volume and maintaining the specific gravity of the suspension within said vessel at a relatively constant set value.

2. In a heavy lmedium minerals separation process in suspension is :transferred to a pump box, and the suspension of magnetic particles, heavier mineral particles sink in the suspension while llighter mineral particles iloat, the resulting sink and float products are successively drained of suspension and washed with water, both the wash water and a variable fraction of the drained suspension are treated to remove -n-onmagnetic contaminants and water and thereby produce a ldensiiied suspension, said densiiied suspension together with :the remainder ofthe drained suspension is transferred to a purnp box, and the suspension feeds from the pump box to a vessel where the mineral particles are introduced, the combination therewith of a method of con-trolling the cleanness of the suspension within said vessel comprising measuring both lthe specic gravity of the suspension Iand the content of magnetic particles therein at a location representative of the vessel, developing a signal proportional to the difference between the specific gravity and the content of magnet-ic particles, which signal is representative of the content of contaminants, introducing water to said vessel in volumes which vary with sai-d signal to maintain a rela-tively constant set content of contaminants in the suspension within said vessel, and adjusting the content of magnet-ic particles i-n the suspension entering said vessel after the water volume changes, Ithereby compensating for changes in the specic gravity of the suspension brought about by variations in the water'volume and maintaining the specific gravity of the suspension within said vessel at a relatively constant set v-aiue.

3. In a heavy medium minerals separa-tion process i-n which mineral particles are introduced to a water suspension of magnetic parti-cles, heavier mineral particles sink in the suspension while lighter mine-ral particles float, the resulting sink and loat products are successively drained of suspension and washed with water, both the wash water and a variable fraction of the drained suspension are treated to remove nonma-gnetic contaminants and water and thereby .produce a densiiied suspension, sai-d densied suspension together with the remainder of the drained suspension is transferred to a pump box, and the suspension feeds from the pump box -to .a vessel where the mineral particles are .introduce-d, the combination therewith of a method of controlling the cleanness of the suspens-i-on within said vessel comprising measuring bot'h the specic gravity of the suspension and the content of magnetic particles the-rein at a loca-tion representative of the vessel, ydeveloping a signal proportional to the difference between the specific [gravity and the content of magnetic particles, which .signal is representative of the content of contaminants, introducing water t-o said vessel in volumes which vary with said signal to maintain .a relatively constant set content of contaminants in the suspension within said vessel, and adjusting both the magnitude of the treated fra-cti-on of the drained suspension :and the rate at which said densied Isuspension is transferred to the pump box in response -to changes in the speci-fic gravity measurement caused by changes in the volume of water to compensate for the change and maintain the speoic gravity of the suspension within said vessel at a relatively constant set value.

4. In -a fluid circuit which handles a water suspension of magnetic particles an apparatus for controlling the cleanness of the suspension comprising means for developing a signal representative of the content of nonmagnetic contaminants in the suspension, means operatively connected with said signal-developing Ameans for introducing a variable volume of water to said circuit to maintain a relatively constant set content of contaminants therein, and separate means for adjusting the content of magnetic particles in the suspension after the water volume changes, thereby compensating lfor changes in -th-e specic gravity of the suspension brought about by variations in lthe water volume and maintaining the spec-itic gravity at a relatively constant set value.

5. In a uid circuit which handles va water suspension of magnetic particles, an apparatus for controlling the clean-ness of the suspension comprising means for measuring both the specific gravity of the suspension and the content of magnetic particles therein, means operatively connected with said measuring means for developing a signal proportional to the difference between the measurements which signal is representative of the content of contaminants, means operatively connected with sai-d signal-developing means `for introduci-ng water to the circuit in volumes which vary with the signal to maintain a relatively constant set c-ontent of contaminants, and separate means operatively connected with said measurin-g means for adjusting the content of magnetic particles in the suspension after the water volume changes, thereby compensating for changes in the specic gravity of the ssupensi-on brought about by variations in the Water vo ume and maintaining the specific -gravity at a relatively constant set value.

6. In a heavy medium minera-ls separation plant, which includes a separati-ng vessel adapted to contain a water suspens-ion of magnetic particles Iin which heavier mineral particles sink while :lighter mineral particles tloat, mean-s for recovering suspension from the sink and -iioat products from said vessel, means for treating a portion of the recovered suspens-ion to remove non-magnetic contaminants and water, and means for recirculating said recovered suspens-ion to said vessel, the combination therewith of an apparatus for controlling the cleanness tot the suspension within said vessel comprising means for developing a signal representative of the content of nonmagnetic contaminants in the :suspension recirculating to said vessel, means operatively connected with said signal-developing means for introducing a variable volume of water to sai-d vesse-l to maintain 'a relatively -constant set content of contaminants in lche ,suspension within said vessel, and means for adjusting the con-tent of magnetic .particles in the suspension entering said vessel after the water volume changes, thereby compensating for changes in the specific gravity of the suspension brought about by variation-s in the water volume and maintaining the specific `gravity of the suspension within said vessel at .a relative- :ly constant set value.

7. In a heavy medium minerals separation plant, which includes a separating vessel adapted to contain a water suspension of magnetic particles Iin which heavier mineral particles sink while lighter mineral particles iioat, mean-s for recovering suspension 'from the sink and float products from said vessel, means for treating a portion of the recovered suspension to remove non-magnetic contaminants and water, and means for reoirculating said recovered suspension to said vessel, the combination therewith of an apparatus lfor controlli-ng the cleanness of the suspension within said vessel comprising means for meas-uring both the specic gravity of the suspension recirculating to said vessel and the content of magnetic particles therein, means operatively connected with said measuring ymeans for developing Ia signal proportional .to the difference between the specic gravity .and the content of magnetic particles, which signal is representative of the content of contaminants, means opera-tively connected with said signal-developing means for introducing water to said vessel in volumes which vary with t-he signal to maintain a relatively constant set content of contaminants in the suspension Within said vessel, and means operat-ively connected with said measuring means for adjusting the content of magnetic particles -in `the suspension entering said vessel after the water volume changes, thereby compensating for changes in the specic gravity of the suspension brought about by variations in the water vol` urne and maintaining the :specic gravity of the suspension within said vessel at a relatively constant set value.

8. In a heavy medium minerals separation plant, which includes a separating vessel adapted to contain a water suspension of magnetic particles Iin which heavier mineral particles sink while lighter mineral particles float, means for successively draining the suspension from sink and iioat produ-cts from said vessel and washing the products with water, means for treating both the wash water and `a variable fraction of the drained suspension to remove nonmagnetic contaminants and water and thereby producing a densied suspension, a pump box .to which are transferred said densied suspension from said treating means and the remainder of the drained suspension, and `a pump for feeding ss-upension from said pump box to said vessel, the combination therewith of an apparatus for controlling the clea-nness of the suspension within said vessel comprising means for measuring both the specic gravity of the suspension fee-ding to said vessel and the content of magnetic particles therein, means operatively connected with said measuring means for developing a signal .proportion to the difference between the specific gravity and the content of magnetic particles, which signal is representative ct the content of contaminants, means operatively connected with said signal-developing means for introducing water to said vessel in volumes which vary with `the signal to maintain a relatively constant set content of contaminants `in the suspension with-in said vessel, and means operatively connected with said measuring means for 4adjusting both the magnitude of the treated fraction of the drained suspension and the rate at which said densied suspens-ion is transferred to said pump boX to compensate for changes in the specific gravity measure ment caused by changes in the volume of w-ater, whereby the speciiic gravity of the suspension within said vessel is maintained at a relatively constant set value.

References Cited by the Examiner UNITED STATES PATENTS 2,760,769 8/1956` `OnSta-d 324-34 y2,965,316 12/1960 Henderson 241-34 3,093,577 6/1963 Wilmot 209-1725 3,235,072 2/1966 Nelson 209-1725 X OTHER REFERENCES Oss et al., Min-ing Eng., vol. 14, No. 5, pages 41-45, May 1962.

HARRY B. THORNTON, Primary Examiner.

FRANK W. LUTTER, HERBERT L. MARTIN,

Examiners.

R. HALPER, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No 3 282 ,4l7 November l, 1966 Paul W. Chase et al It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 62, for "for" read of column 5, line 27, for "supplied" read supplier column 6, line l0, for "suspension is transferred to a pump box, and the" read whic mineral particles are introduced toa water line 7l after "particles" insert a comma; column 7, line I6, after "measurements" insert a comma; column 8, line 34, for "proportion" read Signed and sealed this 5th day of September 1967ia (SEAL) Attest:

ERNEST W. SWIDER EDWARD I. BRENNER Afmeting Officer Commissioner of Patents 

1. IN A HEAVY MEDIUM MINERALS SEPARATION PROCESS IN WHICH MINERAL PARTICLES ARE INTRODUCED TO A WATER SUSPENSION OF MAGNETIC PARTICLES, HEAVIER MINERAL PARTICLES SINK IN THE SUSPENSION WHILE LIGHTER MINERAL PARTICLES FLOAT, THE RESULTING SINK AND FLOAT PRODUCTS ARE SUCCESSIVELY DRAINED OF SUSPENSION AND WASHED WITH WATER, BOTH THE WASH WATER AND A VARIABLE FRACTION OF THE DRAINED SUSPENSION ARE TREATED TO REMOVE NONMAGNETIC CONTAMINANTS AND WATER AND THEREBY PRODUCE A DENSIFIED SUSPENSION, SAID DENSIFIED SUSPENSION TOGETHER WITH THE REMAINDER OF THE DRAINED SUSPENSION IS TRANSFERRED TO A PUMP BOX, AND THE SUSPENSION FEEDS FROM THE PUMP BOX TO A VESSEL WHERE THE MINERAL PARTICLES ARE INTRODUCED, THE COMBINATION THEREWITH OF A METHOD OF CONTROLLING THE CLEANNESS OF THE 